1. Field of the Invention
This invention relates to multitone pushbutton dial arrays and, in particular, to phase reversal scanning circuitry for detecting the operation of a pushbutton switch in the dial array.
2. Description of the Prior Art
Multitone pushbutton dials have been used in telephone station sets for several years. One such dial utilizes a crossbar type switch arrangement and inductor-capacitor oscillator circuitry for generating the multitone signals. This type of dial is mechanically complicated and, therefore, tends to become relatively unreliable after repeated usage. Moreover, this type of dial is somewhat bulky and expensive since the inductors must meet relatively stringent tolerances.
Another type of dial utilizes resistive-capacitive thin film osillator circuitry in conjunction with operational amplifiers and resistive contact switches. This type of dial suffers the disadvantages of requiring expensive, precision circuit components and, because of the use of operational amplifiers, tends to consume somewhat more power than the crossbar type dial.
The use of precision circuit elements for the generation of the multitone signals can be advantageously circumvented by digitally synthesizing the multitones in accordance with a technique disclosed in U.S. Pat. No. 3,787,836, issued to D. W. Hagelbarger on Jan. 22, 1974. However, the replacement of the precision analog oscillator circuitry with digital circuitry tends to make power consumption dependent to some extent on the circuit operating speed.
The problems inherent in resistive contact type switches can be advantageously overcome by the use of capacitively coupled pushbutton switches of a type disclosed in my U.S. Pat. No. 3,904,887, issued Sept. 9, 1975 entitled "Snap-Action Pushbutton Switch" and assigned to Bell Telephone Laboratories, Incorporated. Unfortunately, the replacement of the resistive switches with capacitively coupled switches gives rise to a further problem, namely, the detection of when a given switch in the array of pushbuttons has been operated.
This detection problem can be overcome by the use of some form of scanning. Scanning techniques are well known in the prior art but the application of these techniques to capacitively coupled switches is a somewhat recent development. For example, the scanning of capacitive switches in a keyboard matrix is disclosed in a published patent application No. B289,523 of J. W. Volpe, published Jan. 28, 1975. However, Volpe's scanning technique requires the use of a relatively high frequency carrier signal on the order of 1 megahertz superimposed on a binary selection signal to interrogate each key. Of necessity, this approach requires a separate carrier signal source along with demodulation circuitry to remove the high frequency burst and shaping circuitry to form a square pulse. This circuitry is required in order to detect the change in capacitance produced by a key closure. While this method of scanning capacitive switches is satisfactory for keyboards used in computer terminal equipment, typewriters and the like, where power consumption is not a major consideration, it is not readily adaptable for use in a telephone dial where power consumption is of major importance. The importance of minimizing power consumption in a telephone dial application becomes clear when one appreciates that many such dials must be powered from a single central office and that these dials may be at locations which are many miles from the central office.
Accordingly, it is one object of the present invention to implement a technique for scanning an array of capacitive switches wherein power consumption is held to relatively low levels on the order of a few milliwatts or less.
Another object is to utilize relatively low frequency logic signals to scan the dial array, the frequency of the scanning signals being on the order of 100 kilohertz or less.
Still another object of the present invention is to simplify the overall complexity of the scan detection circuitry.
A further object is to configure a telephone dial utilizing the aforementioned scanning technique so that a number of such dials may be powered directly from a single power source located at a distant central office.